1. Technical Field
The invention relates to an upstream unit for a switched power amplifier of a high-frequency transmission circuit. An upstream unit according to the invention may be used in wireless communication, in multiband and in multistandard radio transmitters, for example.
2. Discussion of Related Art
Switched power amplifiers typically include an amplifier, which is an active circuit, and a reconstruction filter. It is possible with such switched power amplifiers to realize low-loss transmission circuits, for example for wireless communication. The switched power amplifier is driven by high-frequency pulse signals. These must have an amplitude that is sufficiently high and sufficiently constant. Pulse-length modulated signals generated by pulse length modulation (PLM) are specifically suitable as pulse signals. A high-frequency pulse-length modulated pulse signal is therefore generated in an upstream unit for a switched power amplifier and supplied to the switched power amplifier. The latter amplifies the pulse-length modulated pulse signal and supplies it to a reconstruction filter. A modulated high-frequency current signal (HF signal), which is emitted from an antenna, is thus produced at the output of the reconstruction filter.
In pulse length modulation, a pulse length is generally controlled by a threshold value. The length of a pulse is meant to be linearly dependent on the threshold value. However, this linearity in pulse length modulation can only be achieved to an approximate degree in real systems.
In an upstream unit for a switched power amplifier according to the prior art, for example, a phase modulated HF signal is converted by a limiting amplifier into an HF pulse signal in the form of a square wave signal. This pulse signal is converted into an HF triangle wave signal by an integrator. A comparator then compares the momentary amplitude of the HF triangle wave signal with a slow threshold value that varies as a function of time. Depending on the result of that comparison, the comparator outputs a high or a low signal level, which means that a pulse-length modulated HF pulse signal is produced at the output of the comparator. The duration of a pulse is thus controlled by the threshold value. The relationship between the threshold value and the length of a pulse in the pulse-length modulated HF pulse signal is only approximately linear, however. To achieve a high degree of linearity, the HF triangle wave signal must be generated with maximum precision. This requires the HF pulse signal of the limiting amplifier to have a very high edge steepness, firstly, which can be achieved by the limiting amplifier to an approximate degree only. It is also necessary that an integrator which converts an HF pulse signal into an HF triangle wave signal is able to tolerate very short rise times of the input signal. Another requirement is that the HF triangle wave signal must itself have a relatively short rise time so that a sufficiently large signal amplitude is achieved within a high-frequency period, in order to be able to compare the amplitude of the triangle wave signal with a threshold value in the comparator. In a real HF transmission circuit, such requirements can only be fulfilled to an approximate degree, which means that the pulse length of the pulse-length modulated HF pulse signal is generally nonlinearly dependent on the threshold value.
Before the pulse-length modulated HF pulse signal generated by the upstream unit is reconstructed by a reconstruction filter as a transmission signal to be sent to the antenna, it needs to be amplified. Switched power amplifier may be used to perform such amplification. In the case of very small or very large pulse widths of the pulse-length modulated HF pulse signal, the switched power amplifier might not reach its full output amplitude at its output. This means that the relationship between the input signal and the output signal of a switched power amplifier is likewise only approximately linear. This further reduces the linearity between the threshold value and the amplified output signal of the comparator, i.e., the amplified pulse-length modulated HF pulse signal.
If, in the case of digital modulation methods used in many wireless communication systems, the degree of linearity in an HF transmission circuit is low, as described above, this may lead to the receiver being unable to detect digital symbols reliably, and hence to bit errors occurring.
The steepness of the HF triangle wave signal may also be dependent on process and temperature variations, and therefore fluctuate heavily, for example when the upstream unit is realized as a microelectronic circuit. This may result, for example, in an integrator used in the upstream unit having to be calibrated for variations in temperature, for example.
The amplitude of the HF triangle wave signal may also change, in upstream units of the kind considered so far, when the carrier frequency of the phase modulated HF signal changes. Such an upstream unit is therefore suitable to a limited degree only for applications requiring different carrier frequencies, such as multiband or multistandard applications.
Switched power amplifiers typically exhibit asymmetric switching behavior as well. That is to say, there are differences in the steepness of the rising and falling edges of the output signal of the switched power amplifier. After reconstruction of the output signal of the switched power amplifier to form an HF transmission signal, this unequal edge steepness may lead to unwanted phase modulation of the HF transmission signal that is emitted by the antenna. Attention is drawn to the fact that the prior art and the problems associated therewith are also discussed with reference to FIG. 1.